1. Field of the Invention
This invention relates to a pressure swing adsorption system for physically separating molecules in gaseous mixtures, and more particularly to a pneumatic circuit control for such a system.
2. Brief Description of the Related Art
Many processes for recovering oxygen and nitrogen from a feed gas such as air are known in the art. One such process is referred to as adsorption. In the adsorption process, the feed gas is fed into a molecular sieve having pores of uniform size and essentially the same molecular dimensions. These pores selectively adsorb certain molecules and not other molecules. Thus, for example, where air is used as the feed gas a molecular sieve in the form of zeolite might be used. The zeolites would adsorb nitrogen, carbon monoxide, carbon dioxide, water vapor and other significant components of air without adsorbing the oxygen. Accordingly, a very pure form of oxygen can be separated and obtained using the adsorption process. Similarly, where a very pure form of nitrogen is required, a molecular sieve with coke adsorbers might be used to adsorb the oxygen from the feed air, thereby producing a purer nitrogen product.
In a typical adsorption system, a plurality of beds are provided with each bed containing a molecular sieve. The plurality of beds is required because when one bed has become fully adsorbed, it will need to be desorbed before it can produce the product required at the purity level required. Consequently, in a conventional system, a valve control system is incorporated which introduces pressurized feed gas into one bed while simultaneously permitting the desorbing of the second bed via an exhaust valve. Thus, when the first bed is fully adsorbed, the valve control system switches so that the first bed is desorbed and the second bed adsorbs. By using this process, a continuous flow of product gas is supplied to a storage tank from the bed undergoing adsorption. The product gas which flows from a bed undergoing adsorption is typically divided such that part of the gas flows to a storage tank while the other part flows through the other bed to desorb that bed via an exhaust valve.
In addition to the adsorption and desorption phases described above, some systems incorporate a balance phase to improve the efficiency of the pressure swing adsorption system. During the balance phase, the pressures in the two beds are brought into balance prior to an adsorption/desorption phase. This allows the system to utilize the energy that is built up in the bed which has just completed the adsorption phase to help pressurize the bed which will next undergo adsorption. The balance phase increases the efficiency of the system since the compressor supplying the feed gas will not have to work as hard to bring the bed undergoing adsorption up to a required operating pressure.
In order to accomplish the above balancing, conventional systems such as those described in U.S. Pat. Nos. 4,449,990 and 4,560,393 require independent valves and pneumatic circuits at the exit end (opposite the compressor side) of the tanks or beds. Accordingly, these systems are costly, require a large number of pressure connections, and have reliability and noise problems due to the large number of valves and associated moving parts that are required. For example, it is known to use five two-way valves in a two bed system to perform the adsorption/desorption phase and the balancing phase. Four of the two-way valves serve as the inlet and exhaust ports for the beds or tanks, while the fifth two-way valve serves as a "balancing" valve located opposite the compressor side of the tanks.
In other known systems, the above phases can be accomplished using two four-way valves, or a four-way valve and a two-way valve, or a three-way valve and three two-way valves.
What the current art of pressure swing adsorption systems lacks is a pneumatic control circuit which requires less valves and has a single control circuit on the compressor side of the bed or tanks for performing both the adsorption/desorption phase and the balance phase, in order to achieve a more economical and reliable control system.